Radiation-emitting devices are suitable as large-area, thin lighting elements. In many applications it is desirable for the radiation-emitting devices to be transparent. On account of their construction, however, this is accompanied by an impairment of their efficiency.
In conventional transparent organic light-emitting diodes (OLEDs), for example, only part of the generated light is coupled out directly. The rest of the light generated in the active region dissipates in various loss channels such as in light guided by wave guiding effects in the substrate, in a transparent electrode and in organic layers, and also in surface plasmons which can be generated at the surface of a metallic electrode. The wave guiding effects can occur in particular as a result of the differences in refractive index at the interfaces between the individual layers and regions of an OLED. The light guided in the loss channels cannot be coupled out from an OLED in particular without additional technical measures.
To increase the coupling-out of light and thus the emitted light power, measures are known to couple out the light guided in a substrate as emitted light. For this purpose, films comprising scattering particles or films comprising surface structures such as microlenses, for instance, are used, for example, on the outer side of the substrate. It is also known to provide a direct structuring of the outer side of the substrate or to introduce scattering particles into the substrate. Some of these approaches, for example, the use of scattering films, are already used commercially and can be scaled up with regard to the emission area particularly OLED lighting modules. However, these approaches to coupling out light have major disadvantages that the coupling-out efficiency is limited to approximately 60 to 70% of the light guided in the substrate, and the appearance of the OLED is significantly influenced since a milky, diffusely reflective surface is produced by the layers or applied films.
Furthermore, approaches are known to couple out the light guided in organic layers or in a transparent electrode. However, these approaches to date have not yet gained commercial acceptance in OLED products. By way of example, Y. Sun, S. R. Forrest, Nature Photonics 2,483 (2008), proposes forming so-called “low-index grids”, wherein structured regions comprising a material having a low refractive index are applied to a transparent electrode. Furthermore, it is also known to apply high refractive index scattering regions below a transparent electrode in a polymeric matrix as is described, for example, in US 2007/0257608. In that case, the polymeric matrix generally has a refractive index of approximately 1.5 and is applied wet-chemically. Furthermore, so-called “Bragg-gratings” or photonic crystals having periodic scattering structures having structure sizes in the light wavelength range are also known as described, for example, in Ziebarth et al., Adv. Funct. Mat. 14, 451 (2004) and Do et al., Adv. Mat. 15, 1214 (2003).
However, those measures, too, all suffer from the defect that they cause the OLEDs to appear diffuse and the actual transparency effect is lost.
A further problem in large-area radiation-emitting devices is that they often have a voltage drop across their total surface area, the voltage drop having an effect on the luminance and thus on the brightness. The light emitted by the radiation-emitting devices is therefore not homogeneous, but rather has differences in luminance.
It could therefore be helpful to provide an effectively transparent radiation-emitting device having improved properties regarding efficiency or homogeneity of the brightness.